The San Andreas Fault is one of the most studied earthquake-generating structures on Earth, but the reason that some sections are anomalously weak, and creep without apparent seismicity, remains poorly understood. Here, we present results from nanoscale (FIB-SEM) 3D microstructural observations of weak (friction coefficient of 0.095) SAFOD clay fault gouge containing serpentinite clasts, recovered from the active Central Deforming Zone at ~2.7km vertical depth. Our nanoscale observations confirm that frictional slip and extreme weakness occurvia deformation of smectite clay that forms a shear fabric within the fault zone. We infer that creep initiates by fracture-controlled, substrate growth of oriented Mg-smectite on R, P and Y shears, followed by clay smearing and ductile flow of an evolving and expanding clay matrix. At the crystal-scale, pervasive sliding occurs along hydrated smectite interlayers and surfaces occupied by exchangeable Mg- and Ca-ions, with slip typically spaced at 3-5 lattice layers apart. We conclude that the strength and seismic behaviour of major tectonic faults at shallow crustal levels evolves as clay fabric develops with accumulated fault slip.
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